Increased risk of thromboembolic disease has been attributed to the use of hormonal contraceptives in both Great Britain and the United States. Other risks, such as aneurysms of large arteries, hypertension, liver disease, carbohydrate metabolism, etc. have not been explored sufficiently for proper evaluation. Some reports indicate that there is no increase in thromboembolic diseases in women taking oral contraception over that seen in pregnant females. Therefore controversy has centered about the balance between acceptable risks and potential benefits. The principal investigator has previously reported that histological and ultrastructural changes take place in all components of the uterine arteries during pregnancy. Extensive morphological alterations were observed in the endothelium, elastic fibers and smooth muscle. The elastic fibers disappeared during pregnancy and reappeared after parturition. The smooth muscle cells were often rich in smooth endoplasmic reticulum and secretory granules during pregnancy. Current studies have shown that elastic laminae of abdominal aorta of guinea pigs develop extensive vesiculation and some fragmentation after progesterone and estrogen injections. It is possible that the changes in arterial walls might be due to the increase in circulating steroids from contraceptive therapy and hormones of pregnancy. It will be our objective to explore the short and long term effects of antiovulants on the abdominal and thoracic aorta, cerebral and uterine arteries of the guinea pig. Special emphasis will be placed on the effects of antiovulants on the elastic tissue, smooth muscle and endothelium of these arteries. The data will be collected by histochemical and electron microscopic observations.